Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.

The twin-spacecraft Vega project was perhaps the most ambitious deep space Soviet mission to date. The mission had three major goals: to place advanced lander modules onthe surface of Venus, to deploy balloons (two each) in the Venusian atmosphere, and, by using Venusian gravity, to fly the remaining buses past the Comet Halley.

The entire mission was a cooperative effort among the Soviet Union (who provided the spacecraft and launch vehicle) and Austria, Bulgaria, Hungary, the German Democratic Republic (East Germany), Poland, Czechoslovakia, France, and the Federal Republic of Germany (West Germany).

Although the landers were similar to ones used before for exploring Venus, the balloon gondolas were completely new French-made vehicles that carried American-French nephelometers to measure aerosol distribution in the atmosphere. The cometary flyby probes, which contained a 120-kilogram scientific package for investigations, were protected against high-velocity impacts from dust particles.

After a successful flight to Venus, Vega 1 released its 1,500-kilogram descent module on 9 June 1985, two days before atmospheric entry. At 61 kilometers altitude, as the lander descended, it released the first helium-inflated plastic balloon with a hanging gondola underneath it. Mass was around 20.8 kilograms. As the balloon drifted through the Venusian atmosphere (controlled partly by ballast), it transmitted important data on the atmosphere back to a network of tracking antennas on Earth.

Balloon 1 survived for 46.5 hours, eventually terminating operations because of battery failure. The lander set down safely on the ground at 03:02:54 UT on 11 June 1985 at 7.2° north latitude and 177.8° longitude, on the night side of Venus in the Mermaid Plain north of Aphrodite, and transmitted from the surface
for 56 minutes. Having deployed before it reached the surface, the soil sample drill
failed to complete its soil analysis, but the mass spectrometer returned important data.

The Vega 1 bus flew by Venus at a range of 39,000 kilometers and then headed for its encounter with Halley. After a course correction on 10 February 1986, the spacecraft began its formal studies of the comet on 4 March, when it was 14 million kilometers from its target. During the 3-hour encounter on 6 March 1986, the spacecraft approached to within 8,889 kilometers (at 07:20:06 UT) of Halley.

Vega 1 took more than 500 pictures via different filters as it flew through the gas cloud around the coma. Although the spacecraft was battered by dust, none of the instruments were disabled during the encounter. Vega 1 collected a wealth of information on Halley, including data on its nucleus, its dust production rate, its chemical composition, and its rotational rate. After subsequent imaging sessions on 7 and 8 March 1986, Vega 1 headed out to deep space.